Add JMH benchmark

[Nashatyrev]

Generally introduces the ability to add JMH benchmarks

For that:

• extract testFixtures module from the test module.
• add jmh source set and jmh build target (similar to the Teku gradle build)
• make jmh module depending on testFixtures
• add a sample GossipScoreBenchmark benchmark.

The benchmark is in Java since JMH doesn't work well with Kotlin. Tried the kotlinx-benchmark suite, but unfortunately it doesn't work with existing JMH IDEA plugin

[ajsutton]

LGTM.